Graphene/nanostructure FET with self-aligned contact and gate

ABSTRACT

A method for forming a field effect transistor (FET) includes depositing a channel material on a substrate, the channel material comprising one of graphene or a nanostructure; forming a gate over a first portion of the channel material; forming spacers adjacent to the gate; depositing a contact material over the channel material, gate, and spacers; depositing a dielectric material over the contact material; removing a portion of the dielectric material and a portion of the contact material to expose the top of the gate; recessing the contact material; removing the dielectric material; and patterning the contact material to form a self-aligned contact for the FET, the self-aligned contact being located over a source region and a drain region of the FET, the source region and the drain region comprising a second portion of the channel material.

FIELD

This disclosure relates generally to the field of field effect transistor (FET) fabrication, and in particular to graphene-based FET fabrication.

DESCRIPTION OF RELATED ART

Graphene refers to a two-dimensional planar sheet of carbon atoms arranged in a hexagonal benzene-ring structure. A free-standing graphene structure is theoretically stable only in a two-dimensional space, which implies that a truly planar graphene structure does not exist in a three-dimensional space, being unstable with respect to formation of curved structures such as soot, fullerenes, nanotubes or buckled two dimensional structures. However, a two-dimensional graphene structure may be stable when supported on a substrate, for example, on the surface of a silicon carbide (SiC) crystal. Free standing graphene films have also been produced, but they may not have the idealized flat geometry.

Structurally, graphene has hybrid orbitals formed by sp² hybridization. In the sp² hybridization, the 2s orbital and two of the three 2p orbitals mix to form three sp² orbitals. The one remaining p-orbital forms a pi (π)-bond between the carbon atoms. Similar to the structure of benzene, the structure of graphene has a conjugated ring of the p-orbitals, i.e., the graphene structure is aromatic. Unlike other allotropes of carbon such as diamond, amorphous carbon, carbon nanofoam, or fullerenes, graphene is only one atomic layer thin.

Graphene has an unusual band structure in which conical electron and hole pockets meet only at the K-points of the Brillouin zone in momentum space. The energy of the charge carriers, i.e., electrons or holes, has a linear dependence on the momentum of the carriers. As a consequence, the carriers behave as relativistic Dirac-Fermions with a zero effective mass and are governed by Dirac's equation. Graphene sheets may have a large carrier mobility of greater than 200,000 cm²/V-sec at 4K. Even at 300K, the carrier mobility can be as high as 15,000 cm²/V-sec.

Graphene layers may be grown by solid-state graphitization, i.e., by sublimating silicon atoms from a surface of a silicon carbide crystal, such as the (0001) surface. At about 1,150° C., a complex pattern of surface reconstruction begins to appear at an initial stage of graphitization. Typically, a higher temperature is needed to form a graphene layer. Graphene layers on another material are also known in the art. For example, single or several layers of graphene may be formed on a metal surface, such as copper and nickel, by chemical deposition of carbon atoms from a carbon-rich precursor.

Graphene displays many other advantageous electrical properties such as electronic coherence at near room temperature and quantum interference effects. Ballistic transport properties in small scale structures are also expected in graphene layers.

While single-layer graphene sheet has a zero band-gap with linear energy-momentum relation for carriers, two-layer graphene, i.e. bi-layer graphene, exhibits drastically different electronic properties, in which a band gap may be created under special conditions. In a bi-layer graphene, two graphene sheets are stacked on each other with a normal stacking distance of roughly 3.35 angstrom, and the second layer is rotated with respect to the first layer by 60 degree. This stacking structure is the so-called A-B Bernel stacking, and is also the graphene structure found in natural graphite. Similar to single-layer graphene, bi-layer graphene has zero-band gap in its natural state. However, by subjecting the bi-layer graphene to an electric field, a charge imbalance can be induced between the two layers, and this will lead to a different band structure with a band gap proportional to the charge imbalance.

A field effect transistor (FET) may be fabricated using graphene, graphite, which comprises stacked sheets of graphene, or nanostructures formed from graphene such as carbon nanotubes, for the FET channel and source/drain regions. Alternately, the FET may be formed using nanowires made of a semiconducting material for the FET channel and source/drain regions. The source/drain regions of the FET require electrical contacts, which may be formed using a contact material. The contacts to the source/drain regions in a graphene/nanostructure FET need to be formed using a contact material with a relatively low contact resistance, in order to obtain good FET operating characteristics.

SUMMARY

In one aspect, a method for forming a field effect transistor (FET) includes depositing a channel material on a substrate, the channel material comprising one of graphene or a nanostructure; forming a gate over a first portion of the channel material; forming spacers adjacent to the gate; depositing a contact material over the channel material, gate, and spacers; depositing a dielectric material over the contact material; removing a portion of the dielectric material and a portion of the contact material to expose the top of the gate; recessing the contact material; removing the dielectric material; and patterning the contact material to form a self-aligned contact for the FET, the self-aligned contact being located over a source region and a drain region of the FET, the source region and the drain region comprising a second portion of the channel material.

In one aspect, a field effect transistor (FET) includes a substrate; a channel material located on the substrate, the channel material comprising one of graphene or a nanostructure; a gate located on a first portion of the channel material; and a contact aligned to the gate, the contact comprising one of a metal silicide, a metal carbide, and a metal, the contact being located over a source region and a drain region of the FET, the source region and the drain region comprising a second portion of the channel material.

Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:

FIG. 1 illustrates an embodiment of a method of fabricating a graphene/nanostructure FET with a self-aligned contact and gate.

FIG. 2A illustrates an embodiment of a deposited channel material on a substrate.

FIG. 2B illustrates a top view of an embodiment of a deposited channel material comprising nanostructures on a substrate.

FIG. 3 illustrates an embodiment of the device of FIG. 2A after gate and spacer formation.

FIG. 4 illustrates an embodiment of the device of FIG. 3 after deposition of contact material.

FIG. 5 illustrates an embodiment of the device of FIG. 4 after deposition of a dielectric material.

FIG. 6 illustrates an embodiment of the device of FIG. 5 after chemical mechanical polishing.

FIG. 7 illustrates an embodiment of the device of FIG. 6 after recessing the contact material.

FIG. 8 illustrates an embodiment of the device of FIG. 7 after removal of the dielectric material.

FIG. 9A illustrates an embodiment of a graphene/nanostructure FET with a self-aligned contact and gate after patterning the contact material.

FIG. 9B illustrates a top view of an embodiment of a graphene/nanostructure FET with a self-aligned contact and gate comprising a nano structure channel material.

DETAILED DESCRIPTION

Embodiments of a graphene/nanostructure FET with a self-aligned contact and gate, and methods of forming a graphene/nanostructure FET with a self-aligned contact and gate, are provided, with exemplary embodiments being discussed below in detail. The channel and source/drain regions of the graphene/nanostructure FET include one or more sheets of graphene in some embodiments, or nanostructures, such as carbon nanotubes or semiconductor nanowires, in other embodiments. A relatively low-resistance contact that is self-aligned to the FET gate may be formed over the source/drain regions of the graphene/nanostructure FET. The contact may be formed from a material having a relatively low resistance, such as a metal, a silicide, or a carbide.

FIG. 1 illustrates an embodiment of a method 100 of fabricating a graphene/nanostructure FET having a self-aligned contact and gate. FIG. 1 is discussed with reference to FIGS. 2-9. In block 101, a channel material 203 is deposited on a substrate. The substrate includes an insulating layer 202 located on a silicon substrate 201, as shown in FIG. 2A. Insulating layer 202 may comprise an oxide material such as silicon oxide (SiO₂) in some embodiments. Channel material 203 may be one or more sheets of graphene in some embodiments, or may include nanostructures, such as semiconductor nanowires or carbon nanotubes, in other embodiments. FIG. 2B illustrates an embodiment of a channel material 203 comprising nanostructures, such as carbon nanotubes or semiconductor nanowires, on the substrate. As shown in FIG. 2B, the nanostructure channel material 203 is formed on insulating layer 202.

Gate 301 is then formed over the channel material 203, and sidewall spacers 302 are formed adjacent to gate 301 over channel material 203, as shown in FIG. 3. In some embodiments, gate 301 may be patterned from a stack of materials including a thin layer of a gate dielectric, such as hafnium oxide (HfO₂), and, on top of the thin layer of gate dielectric, a gate metal such as titanium nitride (TiN) or tungsten (W), and, optionally, on top of the gate metal, a gate hardmask, such as silicon nitride (SiN). Sidewall spacers 302 may comprise a nitride or an oxide in some embodiments, and may be formed by deposition of the spacer material and etching to form the sidewall spacers 302.

In block 102, a contact material 401 is deposited over the device 300 of FIG. 3, resulting in the device 400 shown in FIG. 4. Contact material 401 may include a metal that has a relatively low resistance, or carbon or silicon in some embodiments.

In block 103, a dielectric material 501 is deposited over contact material 401, as shown in FIG. 5. Deposition of dielectric material 501 may be directional in some embodiments, i.e., dielectric material does not form on the horizontal portions of contact material 401 on sidewall spacers 302, as shown in FIG. 5. In other embodiments, if deposition of dielectric material 501 is not directional, any portions of dielectric material 501 that is deposited on the horizontal portion of the contact material 401 on sidewall spacers 302 is be removed using, for example, a hydrofluoric wet etch, as shown in FIG. 5. Dielectric material 501 may be a material that chosen such that contact material 401 is removable without removing dielectric material 501; dielectric material 501 may include high density plasma (HDP) oxide in some embodiments.

In block 104, chemical mechanical polishing (CMP) is performed to remove the top portion of dielectric material 501 and contact material 401, exposing the top of gate 301, as is shown in FIG. 6. Then, in block 105, the contact material 401 located adjacent to sidewall spacers 302 is recessed below the level of dielectric material 501, as shown in FIG. 7. In embodiments in which contact material 401 includes carbon and dielectric material 501 includes HDP oxide, contact material 401 may be recessed using oxygen plasma. Use of oxygen plasma allows removal of contact material 401 without removing dielectric material 501 or sidewall spacers 302.

In block 106, the dielectric material 501 is removed as shown in FIG. 8. Dielectric material 501 may be removed by any appropriate method. Then, in embodiments in which contact material 401 comprises carbon or silicon, the contact material may be converted to a metal carbide or a metal silicide in order to reduce the contact resistance of contact material 401. The conversion may be accomplished by depositing a layer of a complementary metal on contact material 401 and annealing the device with the deposited metal layer to a temperature that is higher than the formation temperature of the desired metal silicide or metal carbide that the metal reacts with the silicon or carbon contact material 401. The annealing causes contact material 401 to be converted into the metal silicide or metal carbide; all of contact material 401 may react with the deposited metal layer in some embodiments. Then, after the anneal, any remaining unreacted metal is removed selectively to the metal silicide or metal carbide.

In block 107, the contact material 401 is masked and patterned to remove any contact material 401 that is located on non-FET regions of the substrate, resulting in self-aligned contact 901 as shown in FIGS. 9A-B. FET 900A-B of FIGS. 9A-B has self-aligned contact 901 and gate 301 formed over channel material 203. Contact 901 is self-aligned to gate 301 and to the source and drain regions, and may comprise a metal, a carbide, or a silicide, having a relatively low contact resistance to the source and drain regions of the FET. Channel material 203 may include one or more sheets of graphene in some embodiments, or nanostructures such as carbon nanotubes or semiconductor nanowires in other embodiments. FIG. 9B illustrates a top view of an embodiment of a FET 900B showing contact 901 over channel material 203, in which channel material 203 includes nanostructures such as carbon nanotubes or semiconductor nanowires. The portion of channel material 203 located underneath gate 301 comprises the FET channel, and the portions of channel material 301 located underneath self-aligned contact 901 form the FET source and drain regions. Self-aligned contact 901 acts as an electrical contact for the source and drain regions of the FET during operation.

The technical effects and benefits of exemplary embodiments include a self-aligned method of forming a FET having a relatively low contact resistance.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 

The invention claimed is:
 1. A method for forming a field effect transistor (FET), the method comprising: depositing a channel material on an insulator layer of a substrate, the insulator layer having a substantially uniform thickness across a semiconductor layer of the substrate, the channel material comprises a plurality of carbon nanotubes configured in a nonparallel arrangement; forming a gate over a first portion of the carbon nanotube channel material; forming spacers, having substantially vertical sidewalls, on and in contact with a second portion of the carbon nanotube channel material adjacent to the gate, the second portion of the carbon nanotube channel material including undoped carbon nanotube channel material; depositing a layer of contact material over the carbon nanotube channel material, gate, spacers, the substantially vertical sidewalls of the spacers, and the insulator layer of the substrate, such that a portion of the layer of contact material conforms to, and is in contact with the substantially vertical sidewalls of the spacers; depositing a layer of dielectric material over the layer of contact material, the layer of dielectric material being deposited using a directional deposition process that deposits the layer of dielectric material on substantially horizontal portions of the contact material without depositing the dielectric material on substantially vertical portions of the contact material partially defined by the substantially vertical sidewalls of the spacers; removing a portion of the layer of dielectric material and a portion of the layer of contact material to expose the gate; recessing a portion of the layer of contact material located adjacent to the spacers to expose at least a portion of a sidewall of the spacers; removing the dielectric material; and patterning the layer of contact material to form a self-aligned contact for the FET, the self-aligned contact being disposed directly on a source region and a drain region of the FET, the source region and the drain region comprising any portion of the carbon nanotube channel material not covered by the gate or the spacers, wherein the patterning comprises removing any portion of the layer of contact material that is not in contact with the FET regions of the substrate such that a sidewall end portion of the carbon nanotube channel material is covered by the remaining portion of the layer of contact material.
 2. The method of claim 1, wherein the spacers comprise one of a nitride and an oxide.
 3. The method of claim 1, wherein the insulator layer comprises an oxide layer, and the channel material is deposited on the oxide layer.
 4. The method of claim 1, wherein the contact material comprises one of a carbon and silicon.
 5. The method of claim 4, further comprising converting the contact material into a carbide or a silicide before patterning the contact material to form the FET.
 6. The method of claim 5, wherein converting the contact material into a carbide or a silicide comprises: depositing a metal layer over the contact material; annealing the metal layer and contact material at a temperature higher than a formation temperature of the silicide or carbide, such that the metal layer reacts with the contact material to form the silicide or carbide; and removing an unreacted portion of the metal layer selective to the silicide or carbide.
 7. The method of claim 1, wherein the dielectric material comprises a material selected such that the contact material may be removed without removing the dielectric material.
 8. The method of claim 1, wherein the dielectric material comprises high density plasma oxide, and the contact material comprises carbon.
 9. The method of claim 8, wherein recessing the contact material comprises use of oxygen plasma.
 10. The method of claim 1, wherein removing a portion of the dielectric material and a portion of the contact material to expose the top of the gate comprises chemical mechanical polishing (CMP). 